1. Field of the Invention
This invention relates to pressure-sensitive copying systems of the kind in which a substantially colorless color former (dye) held within microcapsules is reacted, upon rupturing of the microcapsules by an applied pressure, with a coreactant material to form distinctive colored marks. More particularly, the present invention relates to improved dye solvents useful in pressure-sensitive copying systems.
2. Description of the Prior Art
In one conventional pressure-sensitive copying system, the microcapsules are carried on one surface of a transfer sheet, referred to as a CB (coated back) sheet and the coreactant material is carried on one surface of a record sheet, referred to as a CF (coated front) sheet. In another embodiment, the microcapsules and the co-reactant material are carried on the same surface of a single sheet. In systems for making a plurality of copies intermediate CFB (coated front and back) sheets are provided. The sheets are usually made of paper.
Most known CB sheets carry a coating of microcapsules, which may be separate or in capsular units, i.e., clusters of capsules. Each microcapsule comprises a wall of hydrophilic colloid material such as gelatin, containing a substantially colorless chromogenic material (color former) of basic reactant chemical properties which, in use, contacts and is colored by a co-reactant material.
The co-reactant material is typically a finely divided acidic compound which is also substantially colorless in its natural form. Commonly used co-reactant materials include organic polymers and inorganic clays which are applied to the CF sheet in a suitable paper coating binder material such as starch, casein, polymer or latex.
Distinctive colored marks occur on the CF sheet following rupture of the microcapsules through localized pressure from writing, typing or printing on the noncoated front surface of a CB sheet which is positioned with its coated back surface in contact with the coated front surface of a CF sheet.
The substantially colorless color former produces color only under acidic conditions, that is, upon contact with the acidic co-reactant of the CF sheet. The color former is always dissolved in a solvent and, in many cases, is diluted with kerosene or the like. It is therefore important that the color former solution possess the required physical and chemical properties.
Generally desirable properties of the color former solution are that it be easily encapsulated by conventional techniques; that it have good shelf life in the encapsulated form; and that it be stable at moderately elevated temperatures. It is also important that the mark produced as a result of the reaction between the color former and the co-reactant develop rapidly, be fade resistant and be resistant to bleeding or feathering as a result of capillary action or other surface phenomena.
The dye solvent (color former solvent) functions to provide a carrier for the color former and a medium for the reaction between the color former and the acidic co-reactant material. The solvent must be capable of holding the color former in solution within the microcapsule, of carrying the color former to the sensitized surface of the CF sheet when the microcapsule is ruptured, and of promoting or at least not inhibiting color development with the co-reactant. In addition, since inadvertent rupture of the microcapsule is possible by careless handling, the solvent must be noninjurious to skin, clothing or environment.
The solvent is an important factor in determining the performance of the pressure-sensitive copying system in terms of stability of the sheets to heat and storage time, rate of color development, extent of color development, and durability of image. Certain prior art dye solvents have exhibited adequate print speed and color intensity on the widely used phenolic resin-coated CF sheets. In some cases, however, objectionable odors in the copying systems have been ascribed to the dye solvent itself. Such odors obviously detract from commercial acceptance of such copying systems even though the dye solvent performance is otherwise superior.
Many non-halogenated aromatic hydrocarbons are known to the art as dye solvents for pressure-sensitive copying systems. Among these are diaryl alkanes, triaryl dialkanes, alkylated biphenyls, alkylated terphenyls, partially hydrogenated terphenyls, alkylnaphthalenes, benzylnaphthalenes and benzyl aryl ethers.
Certain esters have been mentioned in the prior art as dye solvents for pressure-sensitive copying systems. Dioctyl phthalate, for example, was disclosed in U.S. Pat. No. 2,646,367 which issued July 21, 1953. Methyl salicylate, also mentioned long ago as a possible dye solvent, was said to exhibit objectionable odor. Dimethyl adipate, diisodecyl adipate, benzyl acetate and butyl benzyl phthalate, like many esters, have been found to be undesirable as dye solvents because those esters must evaporate before they will permit the color to form. It has been found for example, that a test solution of diisodecyl adipate as a dye solvent had still not formed a color on a receiving sheet after three years of delay. It has further been found that ethyl acetate as a dye solvent totally inhibits color formation because, it is theorized, the ester groups tie up the acid sites on the clay or phenolic resin receiving sheet through hydrogen bonding.
Dibenzyl glutarate, although having good solubility properties, is too slow in print speed. Phenyl acetate and benzyl butyrate are likewise too slow.
In general, therefore, esters have not achieved widespread adoption as dye solvents for pressure-sensitive copying systems even though many esters are in plentiful supply as articles of commerce.
It is an object of the present invention to provide ester dye solvents for pressure-sensitive copying systems which have acceptable odor characteristics yet possess adequate print speed and color intensity together with otherwise suitable properties. Further objects of this invention will become apparent from the following description and examples.